ICU · Renal and metabolic

Acute severe hyponatraemia: emergency correction, ODS risk, and guidelines

Also known as Hyponatraemia · Acute hyponatraemia · Severe hyponatraemia · Hypotonic hyponatraemia · SIADH · Cerebral salt wasting · Osmotic demyelination syndrome · Central pontine myelinolysis

Hyponatraemia (Na <135 mmol/L) is the commonest electrolyte disorder in hospitalised patients. SEVERE (<120) and ACUTE (<48h) hyponatraemia cause cerebral oedema (seizures, coma, death). CHRONIC (48h) hyponatraemia risks OSMOTIC DEMYELINATION SYNDROME (ODS / central pontine myelinolysis) if corrected too rapidly. CORRECTION RATES (critical): severe symptoms → 3% hypertonic saline bolus (Na rises 4-6 mmol/L), then STOP; max rise ≤8 mmol/L in any 24h (chronic), ≤10-12 mmol/L (acute). NEVER give 100 mL 3% saline as a single rapid bolus without reassessment. Workup: plasma osmolality (hypo = <275), urine osmolality (100 = ADH active), urine Na (SIADH 30, hypovolaemic <20), volume status. The BRAIN adapts to chronic hyponatraemia (extrudes osmolytes) — too-rapid correction → brain shrinks → myelinolysis.

Checked against its sources on 2 Sept 2026